Field of the Invention
The invention concerns an arrangement to generate the basic magnetic field and a gradient magnetic field of a magnetic resonance tomography system, a magnetic resonance tomography system with such an arrangement, and a method to operate a magnetic resonance tomography system with such an arrangement.
Description of the Prior Art
Magnetic resonance tomography (MRT) is a medical diagnostic method with the possibility to create slice images of the human body in arbitrary spatial orientations, the slice images showing selected anatomy without other anatomy superimposed thereon. Due to the high resolution and the high tissue contrast, a better depiction of the anatomical structures is achieved than with most other imaging methods. In particular, different soft tissue structures can be better differentiated.
In contrast to computed tomography (CT), x-rays are not used. Instead, the images are calculated from received electromagnetic signals that result from the interaction of hydrogen protons with a strong magnetic field. Hydrogen protons are very common in the human body. Conclusions about the chemical composition, the shape, the perfusion and pathological variations of the examined tissue can be obtained from the response of the hydrogen protons upon radiation of radio-frequency pulses (radio waves in the VHF range) and alternating magnetic fields.
An imaging magnetic resonance tomography system normally has four primary components:
A basic field magnet that generates a static, homogenous basic magnetic field across the measurement volume, so as to align or polarize nuclear spins in an examination subject situated in the measurement volume. Such magnets are executed as permanent magnets, electromagnets or as superconducting coils through which current flows. The direction of the stationary magnetic field is generally designated as the z-component in an orthogonal coordinate system. The other components are designated as x and y.
A radio-frequency transmission/reception device that generates a radio-frequency field in the examination subject so as to excite the aligned nuclear spins by suitable frequency selection. This device can likewise detect RF fields generated by the excited nuclear spins, as magnetic resonance signals (data).
A gradient coil system that is activatable to generate a magnetic field in a direction corresponding to that of the static magnetic field. The strength (amplitude) of the gradient magnetic field varies across the measurement volume. In the normal case, this change is a linear change along a spatial axis. The gradient coil system serves to generate magnetic gradient fields. The independent combination of three gradients that change in strength along three orthogonal spatial axes allows an arbitrary gradient direction to be set, as a combination of the gradient fields. A spatial encoding of the signal received with the radio-frequency transmission/reception device is implemented with the use of these gradients.
A central control unit that regulates the timing of the fields generated in a measurement (data acquisition sequence) and that processes the received signals.
Such a magnetic resonance tomography system is described as an example in DE 10 2008 018 265 A1.
In known magnetic resonance tomography systems, the complicated and expensive superconducting basic field coils and the permanently active basic magnetic field are disadvantageous.